In CT X-ray imaging of a patient, X-rays are used to image internal structure and features of a region, an “imaging region”, of the patient's body. The imaging is performed by a CT scanner comprising an X-ray source that provides an X-ray beam and an array of closely spaced X-ray detectors facing a region, referred to as a “focal spot”, of the X-ray source from which the beam emanates. The X-ray beam is a substantially “fan-shaped” X-ray beam if the scanner acquires at any given time data for imagining only a single “slice” or a small number of slices of the imaging region of a patient. The beam is a “cone-shaped” X-ray beam if the scanner acquires data for imaging a relatively large number, generally more than about 10, slices of a patient's imaging region. A scanner comprising a fan beam is generally referred to as a single slice scanner and a scanner comprising a cone beam is generally referred to as a multislice scanner. Many of the modern day scanners are multislice scanners and the discussion hereinafter generally refers to multislice scanners with obvious modifications where necessary to encompass single slice scanners.
The X-ray source and array of detectors are mounted in a gantry so that an imaging region of a person being imaged with the CT scanner can be positioned in a field of view (FOV) of the scanner that is located within the gantry between the scanner's X-ray source and detector array. When the imaging region of the patient's body is located in the FOV, the X-ray source is operable to provide X-rays that pass through the imaging region and are incident on the detectors. The patient is usually supported lying on a couch that is moveable axially along an axis, conventionally the “z-axis” of a Cartesian coordinate system, relative to the gantry to position and move the patient axially through the FOV. The X-ray source is rotatable around the z-axis. The size and location of the FOV is defined by a largest circle in a plane perpendicular to the z-axis that has its center on the z-axis and for which trajectories of X-rays from the X-ray source that are detectable by the detector array are substantially tangent to the circle. For convenience of presentation and visualization a circularly cylindrical region within the X-ray beam having its axis coincident with the z-axis and a cross section coincident with the largest circle is referred to as a CT scanner's FOV.
In many multislice CT scanners the detectors in the detector array are generally configured in rows and columns of detectors positioned on a circularly cylindrical surface having an axis that is parallel to the z-axis and passes through the X-ray source focal spot. Conventionally, the columns are parallel to the z-axis and the rows lie along arcs of circles that are perpendicular to the z-axis. Features of the cone beam and detector array are conveniently located with respect to a sagittal plane and a transverse plane. The sagittal plane is a plane that contains the z-axis and passes through the X-ray source focal spot. The transverse plane is a plane that passes through the X-ray source focal spot and is perpendicular to the z-axis. A location of a given column is conveniently defined by an azimuth angle. The azimuth angle is an angle that a plane containing the column and passing through the focal spot makes with the sagittal plane. A given row is conveniently located by a declination angle. A declination angle is an angle that a plane containing the row and passing through the focal spot makes with the transverse plane. The dimensions of the cone beam and detector array are generally matched so that X-rays from the X-ray source are substantially confined within a solid angle subtended by the detector array at the focal spot.
To image features and organs in an imaging region of a patient, the couch supporting the patient is moved relative to the gantry along the z-axis to translate the patient's imaging region through the scanner's FOV. As the imaging region moves through the FOV the X-ray source is rotated around the z-axis to illuminate thin “slices” of the imaging region that are substantially perpendicular to the z-axis with X-rays at a plurality of different view angles. At each view angle and different axial positions along the z-axis of the imaging region, detectors in the array of detectors measure intensity of X-rays from the X-ray source that pass through slices of the imaging region. The intensity of X-rays measured by a given detector in the array of detectors is a function of an amount by which X-rays are attenuated by material in a slice of the imaging region along a path length, hereinafter “attenuation path”, from the X-ray source, through the imaging region slice to the given detector. The measurement provides information on composition and density of tissue in the imaging region slice along the attenuation path.
In some CT scanners an axial scan of a patient is performed in which the patient is moved stepwise along the z-axis to “step” the imaging region through the FOV. Following each step, the X-ray source is rotated through 360 degrees or about 180 degrees to acquire attenuation measurements for slices in the imaging region. In some CT scanners a “spiral scan” of a patient is performed in which the patient is steadily translated through the gantry while the X-ray source simultaneously rotates around the patient and attenuation measurements for slices in the region are acquired “on the fly”.
The attenuation measurements for slices of an imaging region of a patient provided by the detectors in an axial or spiral scan are generally processed using CT reconstruction algorithms known in the art as filtered back projection algorithms to map the absorption coefficient of the imaging region as a function of position. The map is used to display and identify internal organs and features of the imaging region.
CT image reconstruction algorithms are used to process attenuation data assuming that for each slice in the imaging region and for each voxel of the slice, attenuation data is acquired for each view angle for an attenuation path that passes through the voxel. To satisfy this assumption the FOV of a CT scanner used to image a patient's imaging region is generally configured sufficiently large to encompass the full width of the patient's body at the imaging region. If portions of an imaging region of a patient cannot fit inside the FOV for all view angles, attenuation data is generally incomplete and artifacts may be generated in images reconstructed from the data. As a result, detector arrays of conventional CT scanners are relatively large and comprise relatively large numbers of detectors.
Typically, a row of detectors in a multislice CT scanner detector array has between 700-1,000 detectors and there may be as many as 64 rows of detectors in the array so that a detector array in a typical CT scanner may have as many as 64,000 X-ray detectors. Future CT scanners are expected to have even larger numbers of detectors. The large number of detectors requires an extensive electronic support infrastructure for signal processing and data transfer. The detectors also require complicated mechanical support systems that are configured to high tolerances that provide in addition to mechanical support, various other functions such as radiation collimation and shielding for electronics associated with the detectors. As a result, CT scanning systems are relatively complicated and expensive.
CT scanning can involve exposing a large part of a patient's body to potentially damaging X-ray radiation. Damage from exposure to X-ray radiation is thought to be cumulative and for safety and health reasons it is desirable to minimize a patient's exposure to X-ray radiation during a CT scan. For some applications for which a region of interest in a patient's body is a relatively localized region inside an imaging region it is possible to limit radiation by limiting radiation exposure to parts of the body that are outside of the localized region of interest. For example, U.S. Pat. No. 6,385,278, the disclosure of which is incorporated herein by reference, provides a method of reducing exposure of a patient to X-rays during a CT scan when a region of interest to be imaged in a patient's body is a relatively small region, such as the heart. The patent describes collimating the beam so that tissue in the patient's body outside the heart receives less radiation than heart tissue.